MONDAY, Sept. 28 (HealthDay News) -- Women under 50 with a certain form of the autoimmune condition called antiphospholipid syndrome are at greatly increased risk for heart attack and stroke, and that risk is even higher when these women smoke or take birth control pills, new research shows.

Antiphospholipid syndrome occurs when autoantibodies attach to cell membranes and interfere with the normal clotting mechanism of the blood.

In the study, researchers in the Netherlands analyzed data on more than 1,000 young women and found that those with a particular type of antiphospholipid antibody, called lupus anticoagulant, are 43 times more likely to suffer a stroke and five times more likely to have a heart attack than the general population of young women.

Women with lupus anticoagulant who smoke or take birth control pills have a much higher risk of stroke (87-fold and more than 200-fold, respectively) and heart attack (34-fold and 22-fold, respectively), the researchers found. Both smoking and birth control pills enhance the action of lupus anticoagulant, they explained.

The study authors estimated that lupus anticoagulant is present in about seven in 1,000 women, but previous studies have suggested a higher prevalence.

The study appears in the Sept. 27 online edition and in the November print issue of The Lancet Neurology.

The findings show that young women with lupus anticoagulant need to be warned about the dangers of smoking and the use of oral contraceptives, Dr. Kathryn Kirchoff-Torres and Dr. Steven R. Levine, of the Stroke Center at Mount Sinai School of Medicine in New York City, wrote in an accompanying editorial.